The present invention relates to an improved method of applying and affixing polymeric compositions to relatively non-porous, hard, substrates, such as glass or glass ceramics, and to the composites produced by the method. More particularly, the present invention involves applying to a substrate having a closed, or heat polished, surface a heat resistant organic resin, which, after curing, provides the substrate with an anti-stick surface. The composites are particularly suited for a wide variety of cookware.
It is well known in the art to employ composites comprising a heat resistant low surface energy organic resin on the cooking surface of cookware. The resin forms a non-stick surface and substantially eliminates the need for cooking oils or fats since the cooked food has a substantially reduced tendency to adhere to the cooking surface. The absence of adhered residue on the cooking surface greatly reduces the time and effort needed to clean the utensil. The addition of fats is usually required only when desired for taste.
The heat resistant, polymeric compositions used in the prior art to coat cookware are aptly suited to use in the present invention. For example, the resin may be a polyamide; a polymer of a halogenated ethylene or a halogenated propylene; or a silicone. Polychlorotinfluoroethylene or mixtures containing preponderant amounts of polytetrafluoroethylene, such as mixtures of polytetrafluoroethylene with other polymerizable materials are particularly useful. When a copolymer is used, it preferably is a copolymer of polytetrafluoroethylene and ethylene. Useful copolymers generally contain at least about 60 percent by weight of polytetrafluoroethylene (PTFE), as well as organic resins operable as binders. A particularly useful polymeric composition consists substantially entirely of polytetrafluoroethylene.
Coatings, or films, of non-stick, heat resistant organic resins, such as polytetrafluoroethylene, are generally applied in a liquid form, usually as an aqueous dispersion. The coating on the substrate is hardened, or cured, generally by heating at, or above, the fusion temperature of the resin and maintaining such temperature for a period of from about ten to about twenty minutes. In the case of typical mixtures of polytetrafluoroethylene and binder resins, the curing temperature will generally be around 380.degree. C. to 430.degree. C. with curing times in the vicinity of about twenty minutes.
Although highly desirable, non-stock coatings on articles of glass or ceramic are produced only with difficulty and such articles have not been produced nearly to the extent of their potential demand. The heat resistant, organic resins which provide the desired non-stick surface do not possess a sufficient degree of adhesion to adhere to a substrate having a closed or relatively non-porous surface. This lack of adhesion results in failure of the coating. Various proposals have been made to remedy the problem. All are expensive and few have been commercially utilized. For example, scoring or grooving of the substrate surface improved bonding somewhat, but results in a weakened product. Sand or grit blasting of the substrate provides a roughened surface which also improves bonding somewhat, but not sufficiently for long term resistance to chemical and mechanical abuse. U.S. Pat. No. 2,562,117 teaches the use of a mixture of polytetrafluoroethylene and chromic acid to overcome the bonding problem. That practice leaves chromate residues which, being toxic, would not receive approval from the Federal Food and Drug Administration for use on articles which will come into contact with food. U.S. Pat. Nos. 3,837,895 and 3,372,053 teach the use of a preliminary layer of glass frit fused to the substrate to provide a bonding surface. U.S. Pat. No. 3,522,075 teaches the use of a preliminary layer of metal oxides deposited on the substrate surface to provide a bonding surface. U.S. Pat. No. 4,311,247 discloses means for sealing glass containers utilizing a membrane composed of an organic thermoplastic film. The process comprises first heating the glass container rim and then treating the rim portion with a thermally decomposable fluorine-containing compound, and thereafter the membrane is pressed against the rim at a temperature above the softening point of the thermoplastic material. British Pat. No. 1,360,222 describes means for improving the abrasion resistance of glass articles wherein the compositions thereof contain sodium ions. The method involves exposing the glass to copper halide vapors to replace sodium ions in the glass surface with copper ions and thereafter coating the surface of the glass with polyethylene, polyvinyl alcohol-carnauba wax, or a silicone resin. [Dutch Patent Application No. 7110549.] U.S. Pat. No. 4,337,295 discloses glasses containing borate-rich and silica-rich phases and observes that the borate-rich phase can be dissolved therefrom. However, there is no discussion to suggest the surface porosity parameters required in the present inventive products to insure strong, secure bonding of the resin to the glass or glass-ceramic substrate.
It is a principal objective of the present invention to provide a method for applying to glass or glass-ceramic substrates a non-stick, heat resistant resin which is bonded to such substrate.
It is a further objective of this invention to bond a non-stick, heat resistant resin on a substrate having a lack of natural adhesion or bonding. The resin is also suitable for use as a base or primer coat for the application of a succeeding layer or layers of the same or other materials.
It is a further objective to provide a composite comprising a base, or substrate, of glass or glass-ceramic having a surface layer of a bonded non-stick, heat resistant resin.
It is a further objective to produce a composite useful for the production of cooking utensils.